His anti-American sermons attracted a wide audience after the US-led invasion in Iraq in 2003, and his reputation rapidly spread.

The BBC's Kim Ghattas, who has interviewed the sheikh, notes that assassinations are highly unusual in Syria.

She says there are a number of stories concerning why he was killed, some of them contradictory, but adds that he does appear to have been instrumental in channelling jihadis into Iraq.

'American agent'

After the shooting, one aide to the cleric told the Associated Press that "terrorists" had killed the sheikh, whose real name was Mahmoud Qul Aghassi, for his "nationalist positions".

The one who carried out the assassination was a prisoner of the American forces in Iraq and had been released some time ago

Sheikh Samir Abu Khashbeh

Another aide, Sheikh Samir Abu Khashbeh, said the gunman had told him that he had killed the cleric "because he [the assassinated cleric] was an agent of the Americans".

"The one who carried out the assassination was a prisoner of the American forces in Iraq and had been released some time ago," Abu Khashbeh said. "He is known to us."

In June 2006, a group of militants killed while attempting to carry out an attack in the capital, Damascus, were found to be carrying CDs of sermons by Abu al-Qaqaa in which he called for US forces in the Middle East to be slaughtered "like cattle".

Afterwards, the sheikh denied he had called on Syrians to go to war in Iraq.

Others have claimed that Abu al-Qaqaa was an agent of the Syrian government, who was used to appease rising anti-American discontent amongst the country's Muslims and to keep the authorities informed of the activities of his fellow jihadists.

Abu al-Qaqaa is said to have kept a low profile in the last year since he was appointed head of a religious school by the Syrian government and he did not openly criticise the authorities.

The Syrian government has yet to make an official statement about the incident.